The present invention relates to improvements in strain gage weighing devices and in particular, to gaged plate transducers characterized by lightweight construction, yet having an increased stability and reliability for use in platform type scales.
U.S. Pat. No. 4,261,429 discloses a platform for a scale in which strain gage transducer beam elements are formed as integral components of the platform. Each strain gage transducer beam is formed in the platform with a flexural free end and a fixed end or root end merging into the plate as part of the platform structure. Strain gages are secured to these flexural transducer beams. Such a platform constructed with strain gage transducer beams as an integral part of the platform are referred to herein as "gaged plate transducers" or simply as gaged plates.
According to the method of preparing gaged plate transducers set forth in U.S. Pat. No. 4,261,429, holes are drilled in the platform in groups of four holes each. A plurality of slots are then machined or otherwise formed into the platform from the side so that the slots extend inwardly from the edges of the platform. Two slots extend in parallel to each other, each joining two holes in order to form a beam structure. Of the four holes one hole is drilled at the fixed end of each slot while the other hole is drilled intermediate the fixed end and the free end of each strain gage transducer beam. All strain gage elements bonded to the beam structures are electrically interconnected to form bridge circuits to provide a weight readout for the scale.
Such gaged plate transducer structures or platforms are incorporated in the weighing apparatus of U.S. Pat. No. 4,411,327 referred to above. Said patent application describes weighing scales with features for minimizing weighing errors due to the effects of horizontal loads temporarily applied to the platform scale. The scales of U.S. Pat. No. 4,411,327 comprise coupling means for transferring vertical load components to the strain gage transducer beams while substantially reducing or preventing the introduction of transverse or lateral load components to the strain gage transducer beams integrally formed in the platform.
According to the apparatus described in U.S. Pat. No. 4,411,327, two parallel horizontal platforms are coupled together in the weighing apparatus. A load is placed on the upper horizontal platform. The strain gage transducer beams are integrally formed in the lower horizontal platform. Force transmitting bars or brackets are secured to the strain gage transducer beams in the lower platform for transferring a vertical load from the upper platform to the respective flexural strain gage transducer beams through a "ball and socket" coupling in which a hardened ball bears against a concave or curved surface of larger radius of curvature than the radius of the ball. Such a coupling of first and second curved surfaces transmits vertical force components from the upper horizontal platform to the strain gage transducer beams while yielding to and substantially preventing the transmission of horizontal force components between the upper and lower horizontal platforms.
The coupling formed by two curved bearing surfaces of different radius of curvature such as a ball and socket coupling is, according to the above disclosure, enclosed in a flexible sleeve of rubber or similar material. The sleeve is preferably secured at its ends to provide a restoring force which dampens lateral oscillations and protects the coupling from contaminations.
U.S. Pat. No. 4,411,327 discloses yet another type of coupling for transmitting vertical force components from the upper horizontal member to the strain gage transducer beams by means of a laterally flexible suspension, such as a cable suspension. Such cable suspensions substantially prevent the transmission of horizontal force components between the upper and lower horizontal platforms.
Gaged plate transducer weighing platforms have many advantages, especially in their low profile, ease of manufacture and accuracy. However, a solid platform has a certain weight and corresponding material cost. The weight and cost factor increases with the weighing range. For example, the single piece gaged plate transducer plate for a scale with a weighing range of one thousand pounds requires an aluminum plate having dimensions of 30.times.30.times.0.5 inches which weighs about 45 pounds and costs over 70 dollars. Furthermore, scales fabricated in accordance with teachings of U.S. Pat. No. 4,261,429 or U.S. Pat. No. 4,411,327 if not constructed properly may be susceptible to torsional force components which may distort proper weight readings.